Proactive uplink transmit power increase in small cells upon outbound handovers

ABSTRACT

The present invention relates to a method and an apparatus for controlling the transmit power of a mobile station (UEX) served by a small cell (C 1 ). In accordance with an embodiment of the invention, the method comprises the steps of, and the apparatus comprises means for, detecting a measurement event anticipating the forthcoming fulfillment of a handover condition towards a neighboring macro cell (C 2 ) by the mobile station, and thereupon step-wise increasing the transmit power level of the mobile station so as to reach a transmit power target (P_Target) when the handover condition is eventually fulfilled by the mobile station. The transmit power target is for compensating for an estimated downlink path loss for the mobile station in the macro cell. The apparatus typically forms part of a radio access point ( 100 ) configured to operate the small cell, such as a femto base station to be operated in residential or business premises, or a pico, metro or micro base station to provide increased capacity in a targeted area of data traffic concentration.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and apparatus for controllingthe transmit power of a mobile station served by a small cell.

TECHNICAL BACKGROUND OF THE INVENTION

With the ever-increasing data traffic demand in today's mobile networks,immediate solutions for capacity improvement are sought by theoperators. Thanks to higher spatial reuse of spectrum, short-radiuscells in the range of 50 to 100 meters appear as a promising solution tosatisfy bandwidth extensive traffic demands and to enhance the Qualityof Experience (QoE) of mobile users.

Heterogeneous Networks (HetNet) are now being deployed, where cells ofsmaller footprint size (so-called pico, metro or micro cells) areembedded within the coverage area of larger umbrella cells (so-calledmacro cells), primarily to provide increased capacity in targeted areasof data traffic concentration. HetNet try to exploit the spatialvariation in user and traffic distribution to efficiently increase theoverall capacity of mobile networks.

Also, with up to 80 percent of the traffic being originated from theindoor where the current mobile networks are least effective due to highbuildings penetration losses, indoor data offloading has become a focusof the industry in the recent years. The indoor data offloadingincentive is significant firstly due to the successful penetration andthe maturity of the fixed broadband technology that can be re-used formobile network backhauling, and secondly because of the substantialcellular network resources that are expended on penetrating buildings.One attractive solution to offload indoor users' traffic is to deployfemto cells (or home cells). Femto cells are short-range cells operatedby subscriber-owned radio access points, and provides improved indoorcoverage and increased throughput to home users while off-loadingtraffic from expensive macro radio access onto the low-cost publicInternet.

Seamless handovers between macro and femto cells is considered as one ofthe major advantages of femto cell technology when compared againstother alternative offloading solutions such as Wifi-based solutions.However, there are some issues that need to be addressed to assuresmooth and successful users' handover from femto to macro cells.

Generally speaking, there are two types of handovers: hard and softhandovers. In hard handover the channel in the source cell is releasedand only then the channel in the target cell is engaged. Thus theconnection to the source is broken before the connection to the targetis made. For this reason such handovers are also known asbreak-before-make handovers. On the other hand a soft handover is one inwhich the channel in the source cell is retained and used for a while inparallel with the channel in the target cell. In this case theconnection to the target is established before the connection to thesource is broken, hence these handovers are called make-before-breakhandovers.

Before explaining the details and respective issues associated to eachhandover mode, the uplink power control algorithms used in Wideband CodeDivision Multiple Access (WCDMA) mobile networks is briefly discussed.

For each activated uplink frequency, the uplink inner-loop power controladjusts the User Equipment (UE) transmit power in order to keep thereceived uplink Signal to Noise and Interference Ratio (SNIR) on thatfrequency at a given SNIR target, SNIR_Target. The base station shouldestimate the SNIR SNIR_Estimate of the received uplink DedicatedPhysical Control CHannel (DPCCH). The base station should then generateTransmit Power Control (TPC) commands and transmit the commands once perslot (i.e., once every 0.66 ms) according to the following rule: ifSNIR_Estimate>SNIR_Target then the TPC command to transmit is “0”, whileif SNIR_Estimate<SNIR_Target then the TPC command to transmit is “1”.

Per 3GPP TS 25.214, there are two algorithms for uplink power control.Each algorithm defines how the TPC commands ought to be interpreted andcombined (when received from multiple base stations). In summary,algorithm 2 is more stable compared to algorithm 1 in a sense that itconsiders five consecutive time slots before making a judgment regardinga change of the transmit power, but is consequently slower thanalgorithm 1. Also, during the soft handover regime, the UE receives TPCcommands from all the cells that it is attached to. However, andregardless of the power control algorithm being used, the TPC combiningprocess is very conservative in a sense that it gives precedence to thebase station requiring the lowest uplink transmit power and yielding theleast interference.

In hard handover, after the UE establishes its connection to the targetcell, it adjusts its transmit power using the open loop power control toestimate the required transmission power for communication with thetarget cell. The UE's initial transmission consists only of the DPCCHtransmission. Per 3GPP TS 25.331 s8.5.3, the UE determines its initialDPCCH transmit power, based on the Received Signal Code Power (RSCP) ofthe pilot channel (CPICH):DPCCH_Initial_power=DPCCH_Power_offset−CPICH_RSCP  (1)

The test set signals a value for DPCCH_Power_offset that places the UE'sinitial transmission near the UE target power setting. The UE targetpower setting is the level that is set for the UE's DPCCH and DedicatedPhysical Data CHannel (DPDCH) transmission. Thus, the test set placesthe initial transmit power of the UE's DPCCH slightly lower than the UEtarget power setting so that when the DPDCH is turned on, the total UEpower matches the UE target power setting.

In classical macro cell deployments, this way of UE's power adjustmentis not problematic because firstly the UE's transmit power is notexpected to change significantly after the handover as the UE is locatedat the edges of the two cells far from both antennas (and most likelyhas been already transmitting on high power to communicate to itsprevious serving cell). Additionally, as the macro cells normally servelarger number of users (compared to the femto cells), the changes in thetransmission power of one single user can not affect the overall uplinkinterference level of other users noticeably. Unfortunately, this is notthe case when considering hard handovers from femto to macro cellsoperated in the same frequency band (or in overlapping frequency bands).Since macro base station is located at much further distance than thefemto base station, the required uplink transmission power to reach themacro base station is significantly higher than the uplink power of thefemto cell users. This implies that when the handover is performed, theUE needs to substantially increase its transmit power level so as tocommunicate with the macro cell. This abrupt and significant change ofthe transmit power introduces a sudden drop of the SNIR for the otherfemto cell user(s).

The aforementioned closed-loop power control mechanisms assures that theother femto cell user(s) can sustain the required SNIR at the femto basestation despite the changes of the radio channel such as signal fastfades due to users' mobility. However, with the very abrupt andsignificant drop of the SNIR, it might take long until the appropriatetransmission power level is reached. This is especially true whenconsidering multiple users simultaneously increasing their transmissionpower (which adds to the overall interference level). The case isdrastically worst if the aforementioned algorithm 2 is used for transmitpower control. Therefore, there is potentially a risk of call dropsduring the adaptation of the users' transmission power. Even if the callcould be maintained, at least severe drop of users' QoE is expected.

Soft handovers may be used too in future femto cells deployment. Duringthe soft handover regime, the UE receives TPC commands from all thecells that it is attached to. However, regardless of the power controlalgorithm being used, the TPC combining process is very conservativesince it is sufficient if the user can at least communicate to one ofthe base stations. Again this is not a problem for traditional macrocell to macro cell handovers as the required transmission power from theedge of one cell to another cell is not varied substantially. Howeverthis is not the case when it comes to soft handovers from femto to macrocells as the user would normally need to transmit with considerablyhigher power to reach the macro base station. In this case, the TPCcommand coming from the femto base station during the soft handoverregime would keep the user's transmission power low and therefore theuser would have difficulty adapting the power quickly enough when he isfully switched to the macro cell (after the handover completes normalpower control is used to adapt the transmission power of the user).Again the case is even worse if the aforementioned algorithm 2 is usedfor transmit power control.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve user mobility fromfemto, pico, metro or micro cells (further referred to as small cells)to macro cells, and to palliate the above shortcomings and drawbacks ofthe prior art.

In accordance with a first aspect of the invention, a method forcontrolling the transmit power of a mobile station served by a smallcell comprises the steps of detecting a measurement event anticipatingthe forthcoming fulfillment of a handover condition towards aneighboring macro cell by the mobile station, and thereupon step-wiseincreasing the transmit power level of the mobile station so as to reacha transmit power target when the handover condition is eventuallyfulfilled by the mobile station.

In accordance with another aspect of the invention, a transmit powercontroller for controlling the transmit power of a mobile station servedby a small cell, and configured to detect a measurement eventanticipating the forthcoming fulfillment of a handover condition towardsa neighboring macro cell by the mobile station, and thereupon step-wiseincreasing the transmit power level of the mobile station so as to reacha transmit power target when the handover condition is eventuallyfulfilled by the mobile station.

The transmit power controller preferably forms part of a radio accesspoint configured to operate the small cell, such as a femto, pico, metroor micro base station.

In one embodiment of the invention, the handover condition is acondition whereby the macro cell is measured as being offset better thanthe small cell by a first positive handover offset, thereby yielding areference receive signal strength or quality threshold in the small cellfor handover towards the macro cell, and the measurement event is ameasurement event whereby a current receive strength or quality levelfor the mobile station in the small cell was measured as being below ahandover anticipation threshold which is the sum of the referencereceive signal strength or quality threshold and a second positiveanticipation offset.

In one embodiment of the invention, the transmit power target is forcompensating for an estimated downlink path loss for the mobile stationin the macro cell.

In one embodiment of the invention, the transmit power target isdetermined by measuring a receive signal strength of a reference pilotsignal broadcast within the macro cell at a nominal downlink transmitpower.

In one embodiment of the invention, the step-wise increase of thetransmit power is a monotonically-decreasing function of a currentreceive strength or quality level for the mobile station in the smallcell.

In one embodiment of the invention, the step-wise increase of thetransmit power corresponds to a linear power increase from an initialtransmit power used by the mobile station in the small cell when themeasurement event is detected up to the transmit power target.

In an alternative embodiment of the invention, the step-wise increase ofthe transmit power corresponds to a second or higher order polynomialpower increase from an initial transmit power used by the mobile stationin the small cell when the measurement event is detected up to thetransmit power target.

In one embodiment of the invention, the increase of the transmit powerlevel for the mobile station is withdrawn if no handover towards themacro cell took place for the mobile station during a handoverconfirmation time period triggered upon detection of the measurementevent.

The present invention proposes to proactively adapt the power of thesmall cell users which are likely to have a handover towards the macrocell soon. A forthcoming handover event towards a neighboring macro cellis anticipated for a particular UE served by the small cell, and theuplink transmit power level of the UE in the small cell is graduallyincreased in such a way that an uplink transmit power target is in forcewhen the handover towards the macro cell eventually takes place.

Typically, the downlink receive power/quality level in the small cell atwhich a handover condition towards the macro cell is fulfilled for aparticular UE is known by means of UE measurement reports. This downlinkreceive power/quality level varies across UEs and from one handoverlocation to another. By averaging this downlink receive power/qualitylevel across multiple UEs, one determines a reference downlinkpower/quality threshold for outbound handovers towards the macro cell.This reference threshold can be used in combination with an anticipationoffset to anticipate an upcoming handover event and to start increasingthe UE transmit power before the actual handover occurrence.

The anticipation offset and the transmit power target are designed so asto reduce the uplink transmit power disruption upon handover execution,and to enable the other small cell user(s) to gradually adapt theiruplink transmit power level to cope with this interference increase andto maintain the required SNIR and hence the call quality.

The transmit power target value is determined so as to compensate for anestimate of the downlink path loss incurred by the UE within the macrocell upon handover execution.

The transmit power target can be determined by measuring the receivesignal strength of a reference pilot signal broadcast within the macrocell at a nominal downlink transmit power.

This pilot signal can be measured by the UE upon detection of thehandover condition, thereby yielding a rather accurate measure of thedownlink path loss when handover towards the macro cell takes place, andthus a rather accurate uplink transmit power target that will avoid anysubstantial uplink transmit power disruption when the UE eventuallyswitches to the macro cell.

Alternatively, the pilot signal can be directly measured by the smallbase station operating the small cell, which is a sub-optimal yet stilladvantageous solution.

Typically, the transmit power increase is a function of a currentreceive strength or quality level for the mobile station in the smallcell. The transmit power increase can be a linear increase or apolynomial/exponential increase from an initial uplink transmit powerlevel up to the uplink transmit power target that is to be achieved.

If no handover towards the macro cell takes place for the UE during ahandover confirmation period, then this ad-hoc transmit power increaseis withdrawn, and the legacy uplink power control algorithm is restoredin the small cell for that UE.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the invention will becomemore apparent and the invention itself will be best understood byreferring to the following description of an embodiment taken inconjunction with the accompanying drawings wherein:

FIG. 1 represents a UMTS mobile network,

FIG. 2 represents a home base station as per the present invention,

FIG. 3 represents a radio coverage area comprising a macro cell and apico cell,

FIG. 4 represents a plot of a receive signal quality for a UE movingoutside a femto cell, as well as respective handover and anticipationthresholds, and

FIG. 5 represents a plot of the UE transmit power controlled as per thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a UMTS mobile network 1 making use of WCDMA radioaccess technology. A UE 11 roams through the mobile network 1. Macrobase stations 21 are provided operating respective macro cells. A numberof such base stations are provided, which are distributed geographicallyin order to provide a wide area of coverage to the UE 11. When the UE 11is within the radio coverage area of a particular macro cell then radiocommunication may be established with the corresponding base stationoperating the particular macro cell over an associated radio linkconforming to the Uu communication interface. Of course, it will beappreciated that FIG. 1 illustrates a small subset of the total numberof UEs and base stations that may be present in a typical mobilecommunication system.

An RNC 31 controls the operation of the base stations 22 and of the UE11 in order to efficiently manage the wireless communication network 10.The RNC 31 communicates with the base stations over a backhaulcommunication link conforming to the Iub communication interface, andfurther with the UE 11 via their respective radio link.

The RNC 31 is operable to communicate with a Core Network (CN) 41, andto route traffic via circuit-switched and packet-switched networks.Hence, a mobile switching Center (MSC, not shown) is provided within theCN 41 with which the RNC 31 may communicate over a communication linkconforming to Iu-CS communication interface. The MSC then communicateswith a circuit-switched network, such as a Public Switched TelephoneNetwork (PSTN). Likewise, the RNC 31 communicates with a Serving Generalpacket radio service Support Node (SGSN, not shown) over a communicationlink conforming to Iu-Ps communication interface. The SGSN is furthercoupled to a Gateway General packet radio service Support Node (GGSN,not shown), which communicates with a packet-switch network, such as theInternet.

In addition, there are provided femto base stations 22, each of whichoperates a femto cell in the vicinity of a building within which theassociated femto base station is installed. The femto cells providelocal communication coverage for a user in the vicinity of thosebuildings. The femto base stations 22 typically utilize a broadbandInternet connection (e.g., xDSL, cable) for traffic backhauling. Thefemto base stations 22 communicate with a femto cell gateway 32 (FGw)via a backhaul communication link conforming to the Iuh communicationinterface.

The femto cell gateway 32 sits between the femto base stations 22 andthe CN 31, and performs the necessary translations to ensure the femtobase stations 22 appear as a RNC to the MSC. The femto base stations 22talk to the femto cell gateway 32, and the femto cell gateway 32 talksto the CN 31 (MSC for circuit-switched communications, SGSN forpacket-switched communications).

The femto cell gateway 32 comprises a security gateway that terminateslarge numbers of encrypted data connections from hundreds of thousandsof femto base stations, and a signaling gateway which aggregates andvalidates the signaling traffic, authenticates each femto base stationsand interfaces with the CN 31.

The femto base stations 22 are low-power, low-cost, self organizing basestations that provide private or public mobile communication services ina residential or business environment. In contrast to current macro cellapproaches where complex and highly reliable base stations are deployedto strategic locations decided by the network owner, the femto basestations may be provided locally by customers for their own use, but arealso a cost effective solution for operators to provide low-cost hotspot coverage and rural coverage.

To reduce the cost of the femto base stations and to reduce complexityand interference effects of the femto cells on macro cells, thetransmission power of the femto base station is relatively low in orderto restrict the coverage area of the femto cell to a range of tens ofmeters or less. The femto base stations have extensiveauto-configuration and self-optimization capability to enable a simpleplug-and-play deployment. As such, they are designed to automaticallyintegrate themselves into an existing macro cellular wireless network.In addition, the femto base stations contain some functionalitiestraditionally provided by the RNC, such as the Radio Resource Control(RRC) functions.

Further small base stations, such as pico, metro or micro base stations,may be provided to operate pico, metro or micro cells. Pico, metro ormicro cells are typically provided by network operators in areas of hightraffic or poor coverage.

There is seen in FIG. 2 further details about a femto base station 100configured to operate a femto cell, and comprising a transmit powercontroller as per the present invention.

The femto base station 100 comprises the following functional blocks:

-   -   a transceiver 110 comprising a digital base-band unit 111 (or        BBU) and an analog band-pass unit 112 (or ANA),    -   a network termination unit 130 (or NTU),    -   a radio resource controller 140 (or RRC), and    -   a transmit power controller (or TPC) 150.

The network termination unit 130 is coupled to the digital base-bandunit 111; the digital base-band unit 111 is coupled to the analogband-pass unit 112; the analog band-pass unit 112 is coupled to anexternal or internal antenna 120. The radio resource controller 140 andthe transmit power controller 150 are coupled to the transceiver 110.The transmit power controller 140 is further coupled to the radioresource controller 150.

Further functional blocks and/or further couplings and interactions thatare not relevant to the present description have been voluntarilyomitted for improved clarity.

The transceiver 110 is configured to establish and operate radiocommunication channels with UEs under control of the radio resourcecontroller 150.

The digital base-band unit 111 is for digitally processing the receivedand transmit data symbols. The digital base-band unit 111 implements thenecessary protocol suites for issuing, terminating or relaying signalingpackets (or control packets), and for relaying user data traffic.

The analog band-pass unit 112 is for modulating, amplifying and shapingthe transmit signal that ultimately feds the antenna 130, and forfiltering, amplifying with as little noise as possible and demodulatingthe received signal from the antenna 130. The analog band-pass 112 unitcan be merged with the digital base-band unit 111 into one single unit.

The network termination unit 130 accommodates the appropriate MediumAccess Control (MAC) and Physical transport (PHY) layers for connectingthrough a broadband connection to a femto cell gateway, as well as someframe dispatching logic for routing the incoming/outgoing frames towardsthe appropriate Input/Output (I/O) ports.

The radio resource controller 140 is for assigning and managing downlinkand uplink radio resources used by the transceivers 110 and therespective UEs for radio communication over the air interface, that isto say a set of code and/or frequency resources assigned to therespective radio bearers for transport of user traffic.

The radio resource controller 140 further configures the active UEs witha measurement policy (see “meas_policy(UEX)” in FIG. 2). Presently, theUEs are configured to periodically report measurements of the servingand neighboring cells (see “meas_report(UEX)” in FIG. 2). Themeasurement report typically comprises Ec/No measurements performed onthe Primary Common PIlot CHannel (P-CPICH) of the serving andneighboring cells. Ec/No stands for the received energy per chip dividedby the power density in the band, and is equal to the Received signalCode Power (RSCP) measurements, meaning the received power on one codemeasured on the P-CPICH of the serving or neighboring cell, divided bythe received wide band power, including thermal noise and noisegenerated in the receiver, within the bandwidth defined by the receiverpulse shaping filter.

The radio resource controller 140 compares the periodic measurementreports with the respective measurement event thresholds so as to detectthat a particular neighboring cell is fulfilling a particular handovercondition.

For instance, a particular handover condition comprises a positiveoffset value OFF1, a Time-To-Trigger (TTT) value TTT1, and possibly anhysteresis value HYS1. The handover condition is fulfilled by aparticular neighboring cell if the receive strength or quality of areference signal from that neighboring cell is measured as beingpersistently better than the receive strength or quality of a referencesignal from the current serving cell by the positive offset amount OFF1and for TTT1 seconds. The hysteresis HYS1 prevents excessive togglingbetween the entering and leaving handover condition.

Upon fulfillment by a particular UE of a handover condition for aparticular target cell, the radio resource controller 140 makes ahandover decision and initiates the necessary signaling exchanges withthe target RNC via the femto cell gateway for carrying out a soft orhard handover of the particular UE from the femto cell towards thetarget cell.

The radio resource controller 140 is further configured to determine areference signal strength or quality threshold HOT for outboundhandovers within the femto cell. The reference threshold HOT isdetermined by averaging the receive signal strength or quality levels asreported by the UEs when a handover condition for an outbound handoveris met by the UEs.

The reference threshold HOT, together with the periodic measurements asreported by the active UEs, are forwarded to the transmit powercontroller 150 for further handling (see “HOT” and “meas_report(UEX)” inFIG. 2).

The transmit power controller 150 is for controlling the uplink transmitpower of the active UEs (see “TPC_cmd(UEX)” in FIG. 2).

The transmit power controller 150 implements the aforementionedclosed-loop power control mechanism designed for maintaining the SNIR ata certain target SNIR.

In addition, the transmit power controller 150 is further configured tokeep track of the current receive signal strength or quality level inthe femto cell for the active UEs, and to detect that the currentreceive signal strength or quality level of a particular UE in the femtocell is past an handover anticipation threshold HOA, which is the sum ofthe reference threshold HOT supplied by the radio resource controller140 and a positive anticipation offset OFF2, meaning that a handovertowards a target cell is likely to take place in the near-future.

Thereupon, the transmit power controller 150 enters an ad-hoc uplinkpower control regime for that particular UE, and issues TPC commands tothat particular UE so as a certain uplink transmit power target P_targetis met when the handover eventually takes place. The ad-hoc uplink powercontrol regime will be further elucidated with regard to FIG. 5.

The uplink transmit power target P_target is determined according to anestimate of the downlink path loss the particular UE is expected toincur within the target cell upon handover execution. P_target can beestimated from UE measurements of the target cell, or from ownmeasurements carried out by the femto base station 100 itself. This isdone similar to the power initialization in the open loop power controlprocedure.

The transmit power controller 150 is further configured to monitor theactual execution of an handover for a particular UE once the ad-hocuplink power control regime is in force for that particular UE.Purposely, the transmit power controller 150 is supplied with a handovernotification message from the radio resource controller 140 whenever ahandover procedure completes for a particular UE, or alternativelywhenever a handover procedure is initiated for a particular UE (see“HO_ind(UEX)” in FIG. 2). The transmit power controller 150 triggers asupervision timer THO once the anticipation threshold HOA is passed fora particular UE, and stops the supervision timer THO when the handoverprocedure completes for that particular UE, or alternatively when thehandover procedure starts for that particular UE. If the supervisiontimer THO expires then no handover took place for that particular UE,and the legacy closed-loop power control mechanism is restored.

There is seen in FIG. 3 a particular radio coverage area of a mobilenetwork comprising:

-   -   a femto cell C1, which is operated by a femto base station BS1,        and    -   a macro cell C2, which is operated by a macro base station BS2.

The cells C1 and C2 may share the same frequency band, in which casesoft or hard handovers are possible between the cells C1 and C2, or maybe assigned non-overlapping frequency bands, in which case only hardhandovers are allowed.

A UE UEX establishes a communication session at position a, within thecoverage area of the femto cell A.

The UE UEX next moves towards position c while the communication sessionis on-going.

At position b, the received pilot signal from macro cell C2 is strongerthan the received pilot signal from femto cell C1. Provided that thedifference between the two received signals' strength is beyond someconfigured handover margin and stays there for a minimum of a TTTperiod, an outbound handover is triggered for handing over the on-goingsession towards the macro cell C2.

There is seen in FIG. 4 the chip-to-noise energy ratio Ec/No of a pilotsignal broadcast within the femto cell C1 and the neighboring macro cellC2 as measured by the UE UEX while it leaves the femto cell C1.

Once the Ec/No value of the femto cell C1 falls below that of the macrocell C2 by at least a positive offset OFF1 value and for TTT1 seconds,the handover occurs. Over the time, the femto base station BS1 iscapable of estimating an average Ec/No level at which handovers aretypically performed. This is denoted by a handover reference thresholdHOT.

To have an accurate estimation of the handover threshold value HOT, thefemto base station BS1 can calculate the moving average of HOT based onthe handover observations over time:HOT(n)←HOT(n−1)+α(HOT(n)−HOT(n−1))  (2),wherein HOT(n) denotes the nth handover observation, and ∝ is anaveraging coefficient (0<∝<1), where higher ∝ values overwrites theolder observations faster.

By introducing an anticipation handover threshold HOA as a potentialhandover threshold, the femto base station BS1 can be prepared to face alikely handover. The anticipation handover threshold HOA is defined as:HOA=HOT+OFF2  (3),wherein OFF2 is a second positive anticipation offset.

The femto base station BS1 monitors the received Ec/No of the activeUEs. Once the value falls below the anticipation threshold value HOA,the femto base station BS1 records the current transmit power of the UEas P_start, and starts commanding the UE to increase its transmit powerlevel so that, by the time of the hand-over, the UE is already on anappropriate power level P_target to communicate to the remote macro basestation BS2. Since the femto base station BS1 can estimate the typicalor average value of Ec/No at which handovers are performed, thethreshold value HOA can be simply calculated by adding an anticipationoffset value OFF2 to this value. Additionally, the target power levelP_target does not need to be accurate as the finer power controlmechanism after the handover (or the power initialization procedureusing open loop power control in case of hard handover) will adjust anypower offset.

There is plotted in FIG. 5 different uplink power control regimes as perthe invention.

Once the UE's Ec/No is below the threshold value HOA, the femto basestation BS1 starts to adapt the UE's transmit power appropriately.

In its simplest form, this power adaptation can be a linear function ofthe receive Ec/No as measured by the UE that results in the uplinktransmit power to be at P_target when the receive Ec/No equalsHOT=HOA−OFF2, that is to say when the handover is expected to takeplace:

$\begin{matrix}{{P\_ UL} = \left\{ {\begin{matrix}{{{{Normal}\mspace{14mu}{Power}\mspace{14mu}{Control}\mspace{14mu}{Regime}\mspace{14mu}{if}\mspace{14mu}\frac{Ec}{No}} > {HOA}}\mspace{65mu}} \\{{{\left( \frac{P_{target} - P_{start}}{{OFF}\; 2} \right) \cdot \left( {{HOA} - \frac{Ec}{No}} \right)} + {P_{start}\mspace{14mu}{if}\mspace{14mu}\frac{Ec}{No}}} \leq {HOA}}\end{matrix},} \right.} & (4)\end{matrix}$wherein P_UL denotes the uplink transmit power. This first option isplotted in FIG. 5 as a plain line.

While this provides a first order linear mapping of the UE's transmitpower from P_start to P_target when Ec/No is decreasing from HOA toHOT=HOA−OFF2 respectively, it might be desirable to have higher ordermapping. This is especially helpful as, in this case, the UE's transmitpower remains lower when compared to the first order mapping. This wouldevidently reduce the total interference level and avoid overprovisioning of all other UE's transmit power (note that all other UEswould respectively adapt their power to sustain their own SNIR values).To account for higher order mappings, equation (4) can be modified as:

$\begin{matrix}{{P\_ UL} = \left\{ {\begin{matrix}{{{{Normal}\mspace{14mu}{Power}\mspace{14mu}{Control}\mspace{14mu}{Regime}\mspace{14mu}{if}\mspace{14mu}\frac{Ec}{No}} > {HOA}}\mspace{101mu}} \\{{{\left( \frac{P_{target} - P_{start}}{{OFF}\; 2^{MO}} \right) \cdot \left( {{HOA} - \frac{Ec}{No}} \right)^{MO}} + {P_{start}\mspace{14mu}{if}\mspace{14mu}\frac{Ec}{No}}} \leq {HOA}}\end{matrix},} \right.} & (5)\end{matrix}$wherein MO denotes the Mapping Order (MO). This second option is plottedas two dashed curves in FIG. 5 with MO set to 2 and 4 respectively.

It is to be noticed that the effective transmit power used by a UE inthe ad-hoc power control regime is a staircase version of the plottedcurves as the UE step-wise adjusts its transmit power upon receipt andinterpretation of TPC commands from the base station(s), that is to sayat specific time instances and at the specified power granularity.

The solution works well with both hard and soft handovers. With hardhandovers, the UE receives the power control commands only from thefemto cell C1 and therefore the power is adapted in a desired way. Withsoft handovers however, the UE receives the power control commands bothfrom the macro cell C1 and the femto cell C2 but will consider the moreconservative power control commands (the one resulting in lower transmitpower), which is very likely to be the femto cell commands.

The solution can be optionally extended to include a timer that isactivated immediately after the received Ec/No of the UE falls below HOAthreshold. If no handover occurs after a pre-defined time periodT_handover, the power control operation can be set back to its normalprocedure. Further, the received Ec/No at the time of expiration of thetimer is recorded as LHOA. This will avoid UE to over-provision thetransmit power if the handover does no occur within a certain timeperiod (e.g., the case when the femto cell user gets close to the femtocell coverage border and does not move from there any further).

If the ad-hoc power control mechanism is cancelled when the receivedEc/No of the UE is below HOA threshold due to expiration of the timerT_handover, it can be reactivated again if:

$\begin{matrix}{{\frac{Ec}{No} < {{LHOA} - {RT}}},} & (6)\end{matrix}$wherein RT is a reactivation threshold.

In other words, the algorithm is reactivated (after being canceled dueto the timer expiration) if the received Ec/No of the UE drops furtherand falls bellow LHOA-RT.

OFF2, T_handover and RT parameters are design parameters that can befine-tuned by the manufacturer.

It is to be noticed that the term ‘comprising’, also used in the claims,should not be interpreted as being restricted to the means listedthereafter. Thus, the scope of the expression ‘a device comprising meansA and B’ should not be limited to devices consisting only of componentsA and B. It means that with respect to the present invention, therelevant components of the device are A and B.

It is to be further noticed that the term ‘coupled’, also used in theclaims, should not be interpreted as being restricted to directconnections only. Thus, the scope of the expression ‘a device A coupledto a device B’ should not be limited to devices or systems wherein anoutput of device A is directly connected to an input of device B, and/orvice-versa. It means that there exists a path between an output of A andan input of B, and/or vice-versa, which may be a path including otherdevices or means.

The description and drawings merely illustrate the principles of theinvention. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention. Furthermore, all examples recited herein are principallyintended expressly to be only for pedagogical purposes to aid the readerin understanding the principles of the invention and the conceptscontributed by the inventor(s) to furthering the art, and are to beconstrued as being without limitation to such specifically recitedexamples and conditions. Moreover, all statements herein recitingprinciples, aspects, and embodiments of the invention, as well asspecific examples thereof, are intended to encompass equivalentsthereof.

The functions of the various elements shown in the figures may beprovided through the use of dedicated hardware as well as hardwarecapable of executing software in association with appropriate software.When provided by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, aprocessor should not be construed to refer exclusively to hardwarecapable of executing software, and may implicitly include, withoutlimitation, digital signal processor (DSP) hardware, network processor,application specific integrated circuit (ASIC), field programmable gatearray (FPGA), etc. Other hardware, conventional and/or custom, such asread only memory (ROM), random access memory (RAM), and non volatilestorage, may also be included.

The invention claimed is:
 1. A method for controlling the transmit powerof a mobile station served by a small cell, wherein the method comprisesdetecting a measurement event anticipating the forthcoming fulfillmentof a handover condition towards a neighboring macro cell by the mobilestation, and thereupon step-wise increasing the transmit power level ofthe mobile station so as to reach a transmit power target when thehandover condition is eventually fulfilled by the mobile station.
 2. Themethod according to claim 1, wherein the handover condition is acondition whereby the macro cell is measured as being offset better thanthe small cell by a first positive handover offset, thereby yielding areference receive signal strength or quality threshold in the small cellfor handover towards the macro cell, and the measurement event is ameasurement event whereby a current receive strength or quality levelfor the mobile station in the small cell was measured as being below ahandover anticipation threshold which is the sum of the referencereceive signal strength or quality threshold and a second positiveanticipation offset.
 3. The method according to claim 1, wherein thetransmit power target is for compensating for an estimated downlink pathloss for the mobile station in the macro cell.
 4. The method accordingto claim 3, wherein the transmit power target is determined by measuringa receive signal strength of a reference pilot signal broadcast withinthe macro cell at a nominal downlink transmit power.
 5. The methodaccording to claim 1, wherein the step-wise increase of the transmitpower is a monotonically-decreasing function of a current receivestrength or quality level for the mobile station in the small cell. 6.The method according to claim 5, wherein the step-wise increase of thetransmit power corresponds to a linear power increase from an initialtransmit power used by the mobile station in the small cell when themeasurement event is detected up to the transmit power target.
 7. Themethod according to claim 5, wherein the step-wise increase of thetransmit power corresponds to a second or higher order polynomial powerincrease from an initial transmit power used by the mobile station inthe small cell when the measurement event is detected up to the transmitpower target.
 8. The method according to claim 1, wherein the methodfurther comprises withdrawing the increase of the transmit power levelfor the mobile station when no handover towards the macro cell tookplace for the mobile station during a handover confirmation time periodtriggered upon detection of the measurement event.
 9. A transmit powercontroller for controlling the transmit power of a mobile station servedby a small cell, and configured to detect a measurement eventanticipating the forthcoming fulfillment of a handover condition towardsa neighboring macro cell by the mobile station, and thereupon tostep-wise increase the transmit power level of the mobile station so asto reach a transmit power target when the handover condition iseventually fulfilled by the mobile station.
 10. The radio access pointcomprising a transmit power controller according to claim 9, andconfigured to operate the small cell.
 11. The radio access pointaccording to claim 10, wherein the radio access point is a femto basestation.
 12. The radio access point according to claim 10, wherein theradio access point is a pico, metro or micro base station.